A fuel cell has been proposed as a clean, efficient and environmentally responsible power source for electric vehicles and for various other applications. In particular, the proton exchange membrane (PEM) fuel cell has been identified as a potential alternative for a traditional internal-combustion engine used in modern vehicles.
The PEM fuel cell includes three basic components: a cathode, an anode and an electrolyte membrane. The cathode and anode typically include a finely divided catalyst, such as platinum, supported on carbon particles and mixed with an ionomer. The electrolyte membrane is sandwiched between the cathode and the anode to form a membrane electrode assembly (MEA). The MEA is often disposed between porous diffusion media (DM) which facilitate a delivery of gaseous reactants, typically hydrogen and oxygen from air, for an electrochemical fuel cell reaction. Plates on each side of the DM and MEA provide reactant and coolant flow, collectively creating a fuel cell. Individual fuel cells provide a relatively low direct-current electrical potential, but can be stacked together in series to form a fuel cell stack that delivers any desired electrical potential. Thus, the fuel cell stack is capable of generating a quantity of electricity sufficient to power a vehicle or to power other applications.
During a typical operation of the fuel cell stack, hydrogen enters and flows through the individual fuel cells from one end of the fuel cell stack to another. The ends of the fuel cell stack are often referred to as the wet and dry ends, with the hydrogen generally flowing from the wet end to the dry end. During periods of non-operation, a quantity of air accumulates in the anode flow fields of the fuel cell stack. Upon start-up of the fuel cell stack, hydrogen is supplied to the anode flow fields. The supplied hydrogen creates a “hydrogen-air front” that locally increases the Reference Hydrogen Electrode (RHE) potential on portions of the cathode that are opposite the air filled portions of the anode. High RHE potential on the cathode electrode can rapidly corrode the cathode electrode, and is known to degrade fuel cell performance.
Importantly, during startup of a fuel cell with high electrical load demand, the non-uniform distribution of hydrogen on the anodes of the fuel cell stack can lead to a phenomenon known as “cell reversal.” Cell reversal occurs when a load is applied to the fuel cell stack and when at least one fuel cell in the fuel cell stack lacks sufficient hydrogen to support a current generated by the other fuel cells in the fuel cell stack supplied with adequate hydrogen. The other cells in the stack cause a locally higher electric potential to that portion of the anode lacking sufficient hydrogen, leading to an oxidation of the carbon support in this region of the anode of the reversed cell that may result in a rapid voltage degradation of the fuel cell, significantly reducing the useful life of the fuel cell stack. In particular, a corrosion of the carbon substrate of the anode electrode of the starved cell, wherein CO and CO2 are formed, occurs.
As some level of hydrogen fill is provided to the cell during startup even for a blocked or flow restricted cell due to compression of gas, a minimum level of charge can be drawn before cell reversal can occur. As a result, a minimum amount of charge may be drawn from the cell without requiring cell voltage feedback. However, fixed resistance loads may not match the minimum charge draw.
In order to mitigate carbon corrosion during startup, known systems have employed a low-impedance circuit to the terminals of the fuel cell stack during start-up. In such systems, a circuit with a low-impedance shorting resistor, for example, is used to minimize the localized cathode electrode potential of the cells in the fuel cell stack. The lower the resistance, the lower the potential observed on the cathode electrode, thereby decreasing the rate of carbon corrosion on the cathode electrode of the fuel cell stack. For the low impedance circuit system to work properly, however, each fuel cell in the fuel cell stack must have substantially equal quantities of hydrogen for the duration of the dead-short, to avoid localized anode starvation in cells deficient in hydrogen. In addition, a low-impedance circuit typically requires costly high current capacity components or else requires some mechanism to slow the hydrogen-air front. The low impedance circuit must also be tuned for each cell, particularly with respect to catalyst area and overall capacitance of the cell.
A fixed resistive load has also been used to suppress stack voltage during startup. However, the fixed resistive load requires the addition of electrical components to engage the fixed resistive load. Further, the resistive load itself adds to the cost and complexity of the system, creating reliability concerns. A fixed resistive load does not allow the electric load to be adjusted based on the needs of the stack or the fuel cell system. For example, some cell voltage monitoring equipment may be powered by the cell voltages, requiring some level of cell voltage to energize the voltage monitoring equipment upon startup.
A number of fuel cell systems and methods are known in the art for optimizing the uniform distribution of hydrogen to the anode flow fields of the fuel cell stack during the start-up operation. Thus, for example, it is known in the art to rapidly purge the anodes of the accumulated air with hydrogen and hydrogen-gas mixtures during startup conditions, to minimize the time that the hydrogen-air front exists on the anodes during startup. The purge is often designed to substantially and evenly fill the anode inlet header with hydrogen without exhausting an excess of hydrogen from the fuel cell system. An illustrative purge method is disclosed in applicant's co-pending U.S. application Ser. No. 11/762,845, incorporated herein by reference in its entirety.
It is also known in the art to control delivery of hydrogen and hydrogen-inert gas mixtures to provide a variable anode flow rate during a start-up of the fuel cell system, wherein the fuel cell system and the method minimize an anode fill time. One such method is disclosed in applicant's co-pending U.S. application Ser. No. 12/725,771, incorporated herein by reference in its entirety. In combination with a dead short, the system described for controlling the delivery of reactants to the anode electrodes allows the electrical current to be varied. However, such a method requires a high precision control of both the delivery of reactants to the anode electrode and application of the dead short.
There is a continuing need for a responsive and controllable fuel cell system and method that protects against localized corrosion within a fuel cell during startup by minimizing the electric potential within the fuel cell without requiring additional components or cost. Desirably, the fuel cell stack and method minimizes the effects of a non-uniform distribution of hydrogen during startup and militates against a voltage degradation of the fuel cell stack.